After the email user activates the address, you can change the password. To do this:
Visit your Email Control Panel.
Click the "Edit User" link for the user you'd like to edit.
Email Control Panel - Edit User Link
Click the "Change password" link.
Email Control Panel - Change Password Link
You may be prompted to enter your security key. Enter the security key and click the Continue button. (If you have never created a security key, you will be prompted to create one.)
Verify Security Key Screen
Enter and confirm the new password, then click the Save button.
Email Control Panel - New Password Fields
Be sure to tell your user you've made this change, as he or she won't be able to access her Business Mail account without the new password.